Tuner for radio receivers



Nov. 29, 1949 w. .1. OBRIEN TUNER FOR RADIO RECEIVERS 5 Sheets-Sheet 1 Filed July 15, 1946 jam.-

Nov. 29, 1949 W. J. O'BRIEN TUNER FOR RADIO RECEIVERS 5 Sheets-Sheet 2 Filed July 13, 1946 Nov. 29, 1949 v w. .1 O'BRIEN 2,489,721

TUNER FOR RADIO RECEIVERS Filed July 15, 1946 5 Sheets-Sheet 3 FREQUENCY IN KCS.

%f/Ch'llt j l f ka'ewz M) M @647] A Nov. 29, 1949 w. J. OBRIEN 2,489;721

TUNER FOR RADIO RECEIVERS Filed July 13, 1946 5 Sheets-Sheet 4 Nov. 29, 1949 w. J. OBRI EN 2,489,721

TUNER F OR RADIO RECEIVERS 5 Sheets-Sheet 5 Filed July 13, 1946 4- 4 4- 4- 4- 4 4- 4 4 4- 4 4- 1- 4 5 5 5 5 5 G 6 h 5 7 3 5 7 9 l 3 QQ/Wr/eve 07/' m/ ze 6 4 Patented Nov. 2 9, 1949 UNITED STATES OFFICE Wil iam .J- @Brien Le oni Englandmssigmr to Edward F- Andr ws, ago, 7 1.1-

Application Jilly 13, 1946, *SeriaLNo. 683,312

3.- onl when a de n te tar ie r ueue .si A f-umber pliieptist co.- bme. the ba a eea p ased sharp y respp siveeireu w th s o a red-i9 re e ver is. pre ee. a.

step tee tuning m at the A fu r object of th to prpszisis issued August 26, 1947 as Patent No. 2,9;26530,

This invention relates tomeans for improving the rapidity of action, one-accuracy of tuninaand the discrimination in favor of. the desired signal in radio receiv rs having stop-.enrcarrier tunin and for otherwise ii DEQViDEI-SHQH devices.

T e inventienffurtner relatesrto .thedrive mesh: anism for regulatin the. volume and effectin st p-on-carr er tn rin autqmebiies t or h me sets, e er from; me e. .ppintznr otherwise.

T e nv nti relat s elects). a n w and im p ov c eu .v rana me tt iwadis riminatins'hetween si na s never-ins broaduanri pa -r rv- :free q ency bands.

One of h ob cts of this inventien is to pre v d an mpr ved ra ii nece. the stop n carrier type.

Another h-J'ect .of this 'inren i is in PfOYiiifi impr ved m ans :fQr s p in pr radiu ti e the freq y of d p appa atus- A her ob ect is. to pr de a ste na ri r g i cuit shal he extrem y sharp in its response so that :the tuning means can-not be stopped ex ept v ry elps towthe p r e t y tuned position.

er object ,isto rovidermeans ior-effieetm sharpl tun d iast ae step-Qn-earrier nalsq ite tuning, actuable by carrier s g narrow frequ ney, and relatively unresp nsiy te broader signals.

A fu r object i t provide a. new nd mproved circuitarrangement QIHGiSQIil'Ii-inafing w n si nals pverin broad. and natit ew fr quency bands.

A further object is to provide a palanced and phased ci cui efieets a ist pne ch n e n the effectiv signal stren h only :Qu e exac y at resonance. v

A furt ob ect 10f 2th nvention :is t EQEQYid-fi a balanced-andphased rapidly as re onance :is appmauhed, and wh n com ined asieznal ,of more near yeen- .stant magnitude a eresenance and of reppes te p o1arity,.causes tire o eration at relay means to age. ogpoitlg ,1. value at a frequency slig V isplaced from the intermed te -reg-uenev a m ni i. ct. of th m ent ntention .prpvide an .impre es radio circuit arrangement for controllingav dc.- rice-. 111 theplatep mu t 9f enelectmn tube a i a r d n response t a signal v ltaeero a made.- t rmipesi frequency. which arr n e ent ce Prises means for biainine item -.t ie;r .c io.a.eeutrol .velta e tn. maximuma sa d freq ency. me ns iorp tamp i em t erame an ep psins control voltage that is a maximum at a mas ge s ehtlydisplacedfrom se sirfreeuene and mii m at s f eguene rand .meapsiileluti uerectiming m ans .1 imp essin a voltage t at is a resul ant p said tw cam Meltaees 1 1 1 tire .erisief saidv tube ea rete .d. oppo tage i l we tha the oth r sen-tria yeliage.

.enqth r ab ect o the p ese ipv ptie "t provide a new and improved frequency discg f Y patipg-eir u s er ceepiin dev e "1 ape ei to a ource o a ternating; purren v l a e .11 ...s

A ..eyiee .QQRQS d z eeetive .oii

rpli-ng sm an ar n ed to p .e zer ea ti e ceup ipe e uips eat said :pnes etentmped .fi equeney- A :iurtper- .olzrieetof the prese t .inrept en is :to

.pmv xie :8 mew aridsimprpye .stepp ieaeLrati re e ve z z spsm re i9 ass n a1 n spp is zv e5 inibrea ziatenseiipien l ie ee eemprism(variabl tuning means, amplifying means associated with said tuning means supplying an alternating current voltage of a limited band width containing both signal and interference, electron tube means operable by an actuating voltage to control said tuning means, and means coupling the amplifying means to said electron tube means for supplying, in response to said signal, and without substantial attenuation, an actuating voltage corresponding to a very narrow band of frequencies near the center of the limited band and preventing the passage of any effective actuating voltage in response to said broad interference, said last mentioned means including highl resonant filter means having comparativel negligible resistance, and voltage suppressing means.

Another object of the present invention is to provide a control including means controlled in secondary or which is connected to a rectifier tube 21 which supplies B current to a B supply line 28.

The receiver is tuned by means of a gang of condensers 29 operable by a. shaft 30. The gang of condensers is adapted to be driven by a reversible motor 3| through pinions 32, 33, 34, 35, clutch elements 38 and 31, and pinions 38, 39, 40, and 41, the latter being rigidl mounted on the shaft 36. The clutch element 31 and the pinion 38 are mounted on an axially movable and rotatable shaft 42 which is normally biased to the left, as viewed in Fig. 1, by a spring 43. The shaft 4'2 is movable to the righ, as viewed in that figure, by an armature 44 (with respect to which it is rotatably mounted) upon energization of a fast acting relay 45 constructed and arranged in circuit relative to a gas type control tube in a response to actuating and restraining voltages wherein I the restraining voltage is supplied through discriminating means attenuating the voltage and including amplifying means for amplifying the output of said discriminating means relative to the actuating voltage to provide a restraining voltage having a maximum voltage substantially higher than the maximum of said actuating voltage and with little suppression of said actuating voltage.

Other objects and advantages of the present invention will become apparent from the ensuing description, in the course of which reference is had to the accompanying drawings, in which:

Fig. l diagrammatically illustrates one embodiment of the invention;

Fig. 1A is a fragmentary enlarged view of a 1:

portion of the apparatus shown in Fig. 1;

Fig. 2 is a graphic representation of certain control voltages obtaining in the embodiment of Fig. l, and in other embodiments;

Fig. 3 is a graphic representation of the resultant of two of the control voltages illustrated in Fig. 2;

Fig. 4 is a diagrammatic representation of another embodiment of the invention;

Fig. 4A is a fragmentary enlarged view of a portion of the apparatus shown in Fig. 4;

Fig. 5 is a partially diagrammatic and. partially perspective View of another embodiment of the invention;

Fig. 6 is a partially diagrammatic view of a further embodiment of the invention;

Fig. 7 is a partially diagrammatic view of a still further embodimentof the invention;

Fig. 8 is an elevational view, partl in section,

of the apparatus shown in Fig. 6; and

Fig. 9 is a graphic representation of a voltage obtainable'with the apparatus shown in Figs. 5and 6.

Referring to the drawings, Fig. 1 depicts a standard automobile superheterodyne receiver to which elements of my invention have been added. The receiver comprises an aerial if), a radio frequency tube H, a first detector and oscillator l2,

an intermediate frequency tube [3, a tube [4 comprising a diode section l5 and an audio amplifier section l6 and two power amplifier :tubes ll connected in push-pull relation. The output of the power tubes 1 l is connected through a transformer I8 to the voice coil [9 of a speaker 20.

Energy is supplied by a battery 2i adapted to be connected by a switch 22, illustrated in circuit closing position, to the cathode heater circuit 23, the speaker field coil 24, and to an interrupter 25. The interrupter controls the fiow of current through .the primary of a transformer 26, the

manner specifically set forth and claimed in my copending application Serial No. 387,908, filed April 10, 1941, now Patent No. 2,442,430 which issued June 1, 1948. The clutch elements 36 and 31 are in engagement when the relay is deenergized, and are disengaged when the relay is energized-the elements being shown in disengagement better to illustrate the construction. The anmature 44 controls the operation of switches 46, 41, and 48. Switches 46 and 48 are closed and switch 4! is open when the rela 45 is deenergized. When the relay 451s energized, switch 41 is closed and switches 46 and 48 are open, as shown in Fig. 1.

The pinion 34 meshes with a gear 49, which is rigidly secured to the pinion 50, the gear 48 and pinion 5!] being mounted on the rotatable armature 50 of a solenoid 5|. The pinion and armature 50 are rotatable about a shaft 5! supported by a suitable bracket and the inner end of shaft 42, which is made hollow so as to receive the shaft 5| and so as to be mov able relative thereto by solenoid 45. When the solenoid 5i is energized, the pinion 5G meshes with a gear 52 which isconnected through gears 53 and 54 to a shaft 55. When the solenoid 55 is deenergized, a spring 56 forces the gear and pinion 49 and 50 to the left, as viewed in Fig. i, so that the pinion 50 is moved out of mesh with the gear 52. It will be understood that a clutch similar to the clutch 36 and 37 may be employed to connect the pinion 58 to the gear 52 instead of the meshing and unmeshing of the gears, if desired. The shaft 55 actuates the movable arm 51 of a volume control potentiometer 53 and also actuates the power switch 22 and a motor circuit controlling switch 59.

The shaft 30 carries an abutment member 60 which is adapted to throw a two-way limit switch 6| from one position to the other at the limits of movement of-the condsensers 23. As will hereinafter be readily understood, the throwing of the switch 6 I, which is connected to switch 59, reverses the directionof rotation of the motor 3| so that the condensers move back and forward from one extreme position to the other, and vice versa, until the motor is stopped when a carrier is received.

The receiver is adapted to be controlled from a remote position by means of a remote control unit which comprises the elements in the ex treme left hand top corner of Fig. l. The remote control unit comprises a normally closed switch 62, a sensitivity controlling rheostat 63,

and a single pole, double throw power and volume control switch 64 which is resiliently biased to a normally open intermediateposition, in which it is shown. One side of the rheostat 53 is connects the. gears 50. and 52 so that the shaft 55 is driven in the volume increasing direction, closing switches 22 and 59. At the same time, the relay 45 being deenergized, the condensers 29 are driven through the clutch 36-31. After the set is turned on, the switch is released and the motor operates first in one direction and then in the other, as determined by the limit switch 6|. In. one direction of operation motor winding II is energized, and in the other,,winding I2 is energized, the windings being alternately connected across the battery through circuits including switch 59, the position limitswitch GI and the grounding switch 46. Upon release of switch 64 the solenoid 5| is deenergized to terminate operation of the volume control.

The backward and forward movement of the condensers continues until the tubes have warmed up and a station is received. When a station is received, automatic volume control voltage is supplied from the diode detector I5 to line I5, from whence it is applied to the grids of the tubes I I, I2 and I3. As a result of the application of this automatic volume control voltage on the grid of the radio frequency tube II, the plate current of the tube is decreased, and the voltage drop across resistance 91 is decreased. Consequently, a more positive voltage is applied to the screen of the pentode section of tube 81 through conductor 95. The voltage thus applied to the screen has a value which follows approximately the contour of curve I02 in Fig. 2 as the signal being tuned in approaches resonance with the I. F. circuits of the receiver, this voltage rising rapidly as a station is being tuned in and reaching a maximum when a station is exactly tuned in. As a result of the increase ofthe voltage on the screen grid, the plate current of the pentode section of tube 81 increases, and the potential drop across the resistance 90 increases. Consequently, a corresponding voltage is applied through resistances 92 and 94 "upon the grid of the gas tube I0. The voltage thus applied to the grid of the gas tube is depicted by the curve I02 on Fig. 2, which indicates the variation of the voltage with respect to frequency.

The primary 16 of the transformer 11, condenser 8I, resistance 80, condenser 19, large bypass condenser It, and the mutual reactance of the condenser 83 constitute a tank circuit tuned to the intermediate frequency. Condenser 19, inductance 8|, condenser 82, together with the mutual reactance of the condenser 83, also constitute a tank circuit tuned to intermediate frequency. This circuit is excited through condenser 19 and also through condenser 83. The coupling of these tank circuits through condenser 19 is inversely proportional to the frequency, while the coupling occurring through coil 8|, which is energized by a portion of the primary tank circuit current flowing through condenser 83, is proportional to the frequency. Due to these opposing electrostatic and electromagnetic couplings and the parameters of the elements of the two tank circuits, when a signal of exactly intermediate frequency is received by the primary I6, the potential across the condenser 82 is a minimum. In other words, the effect of the coupling occurring through condenser 83 produces a voltage drop across the inductance 8| which is equal and opposite to the voltage drop occurring across the condenser 19 at resonance. The value of the resistor 80 is selected so as to bring the voltage across the condenser I9 one hundred and eighty degrees out of phase with the voltage across the coil 8|. Under these conditions, there will be substantially zero voltage across the condenser 82 at resonance. Considered in another light, the electromagnetic and electrostatic couplings are balanced or neutralized, i. e., the inductive or capacitative reactive components are made to balance or neutralize. There remains a resistive component of coupling which can be balanced or neutralized only by a resistive component supplied, in the arrangement described, by the resistor 80. In one aspect the arrangement of the coupled circuits maybe considered as a balanced bridge circuit or a circuit providing zero coupling at resonance.

On the other hand, if the signal received by the primary I6 isslightly out of resonance, there is no longer a balance between the opposing couplings; that is, the effect of one coupling is greater than thatof the other, and a potential varying with the departure from the resonance frequency exists across the condenser 92. The variation of the voltage across the condenser 82 with frequency is indicated diagrammatically in the curve I03 of Fig. 2. Thus, it will be seen that when a station is accurately. tuned in, no signal (or a minimum signal in case the balance between the two tuned circuits is not quite exact) is applied to the control gridBB of tube 81. When the variable condensers 29 are approaching exact tuning, that is, two or three kilocycles away from exact tuning, a very large alternating current voltage is applied to the grid 86 and a correspondingly large alternating current voltage is induced in the secondary of the transformer 95. This voltage is rectified by the diode section of tube Bl and-causes a voltage drop across resistance 92, the plate or negative side of which is connected to the grid of the gas tube through resistance 94 to applya negative voltage to that grid. The variation of the negative voltage applied to the grid of the gas tube with respect to frequency is shown by the curve I04 of Fig. 2. It will be understood that the curve I04 generally corresponds to the curve I03 after amplification in the pentode section and rectification by the diode section 88 of tube '81. The height or magnitude of the single peaked relative to the double peaked control voltage is controllable by varying the coupling of the transformer 95.

Referring again to Fig. 2, it will be seen that a resultant voltage equal to the algebraic sum of the positive direct current voltage represented by the curve I02 and the negative direct current voltage represented by the curve I04 is applied to the grid of the gas'tubelfl. The resultant voltage applied to the grid of the gas tube is definitely negative more than one kilocycle on each side of perfect tuning. This resultant voltage is depicted by the curve I05 in Fig. 3. When resonance is more closely. approached, the positive voltage shown by curve I02 predominates, and the grid of the gas tube 10 becomes sufiiciently positive to overcome the fixed negative bias on the grid of the gas tube resulting from the voltage drop across the resistances 69 and 63. The voltage at which ionization occurs may be some value, such for instance as that indicated by the dotted line min Fig. 3, so that the gas tube is ionized only when the tuning means condensers are in position very closely to tune in a station. The tube 10 ionizes and its plate-cathode resistance drops to a low value. The condenser 98, which accumulates a high charge by reason of its connection to the B supply through the resmear 1sistorifi9 during edeionizationsoi stubs E10,;now discharges :napidiy itl'fllfillgh s'th'e rrelay sand the clutch :.elements I36 :and it! instantly disengage, thus terminating theohr'rveaoiatheicondensers. .At :the sazmetimegthesswitch tfidsppenedz-to disconmeet *the .motor from the .cbattery. flhe steady .currentzflow through (the resistor Qfizand relay d5 suffi'cient.toYholdatherelayeclosedaatter=the con- .denser s98 :is --discharged. As ?the .rswitch 46 :is opened, the switch &8 zis ieilso opened. 'This disecnnnects the connectionbetween-the:grids of the etubes I] and groundwh ichlha'd previously muted "theaudio irequencycoutput of thezset. .Thus, the audio. output :of theset canvagain heard from sthespealser l9. Ainntl'rer'hnnction.pmfiormed by 'the relay M is :170 zclose -.=the switch 141., ithus -=con- -necting one -,:=terminal not :the :condenser M, which was previously open ucircuited, zto ground. This renders the condenser YMefiective to increase the itime delay: applied :to therautomatic volume conttml uvzoltage whichaismequired ioryproperguality :rluring the siaud-iblerreception ofia signal.

When the wcondenser '14 .:is :not in .circuit, as when theLtu-ning-ope-rationdieing effected, the

:automatic gain zcontrol has :a time constant --shorter than the time required for the variable tuning means togpass by a radioisignal. .This in- :sures :that the :arctuati-ng voltage will build up suiilciently ltast :to effect ionization 'of the gas tube when the station is tuned 3111, thus prevent- :ing .a meduction in sensitivity which would result g-fr m-. ,.-.de1a;yi;long: as 'zcompa-red-lwithltime of passing througlvthesignal. .Elh-is is important when :theactua-ting voltage is derived, as here, from :the automaticzgaimeontilol. Inrbetween stations :the'automatic; gainvcomtrol r-yoltage is a minimum and if delayed the automatic .gain control voltage would notacomeupetmaz-maxinrum until after some delay until editor the station -.-had:been tunedingii thedelaywas :suflicie-ntly long. However, :by utilizing \the fast acting automatic :gain .control-the.-gain control Lvoltageris bniltu-p :rapidly, .as @is the actuating voltage. Consequentlygthertuning in of .the signal isinsured.

The volume imay the adjusted eat-any :time by linov-ing'the movable =.blade of .thervolume control switch .5 3 to -.-one .side .or .theiot-her. Thesolenoid 5| is thereby energized loperatively connecting the motor to thesh-a-ft 5 5,, and the-motorisenerv-gized xandvdrives .shaitfifi to unove the .arm 51 in the appropriate direction to increase or decrease thei/oln-me.

when it lie-desired totu'nein a difierent staitionthe switchitzis openedmanually-sothat the cathode .to ground .vconnectionaoi the p late :circuit .of the gasttube llflr-isihroken. Thegastube is thus deionised and the melay-t5 is:consequently deen- :ergized, and the switch A6 closes so that the .motor .is .put into operation. When the switch it? is allowed-to close againrthegas-tube lativill re-ionized when asignazl ofadeguate strength is received.

The grid biastoi sthe gas tube 1.6 maybeactiusted at will byrmeanssofctheerheostatifidso-:that the minimum strength oi station .to :which the stop-on-carrier.systemwillrespond may-be regulated-atwill.

It may be -well briefly to reviewlsomel-of the outstanding advantages r-Of the balanced bridge orzero couplingat resonance r-circuitsfor stop-oncarrier tuning. Elhis circuit-produces a-double ,pealiedtselectivityrcurye, i305 shown at I 94 .in Fig. 2, the. signal yoltageof which drops very sharply in the immediate wici-nity-loi resonance. This very sharp change in signal strength with-(small. v;

10 :ireqnency .vaxriationsris not dependent on-the r orous eliminationlnfrresistzmce ingthe circuit, but

unherenttin theeircuit arrangement. it :provides :the .emremelysharp selectivity desired Joy :simple and .inexpensivemeans. The phasing or balancing {out :of the effective :resis-tance in :the balanced-bridge ircuitproduces a tyre of resonance curve in-which-lthe sides-of the depression :;between the two weeks are :straight and steep, seeming practiealiy to a pointatthe bottom so :th-atvol-tage varies substantially linearly with respect .to frequency. curve is the v-selectivity -.-cur-ve of a ,-tunedecireuitfwith practically zero :resistance, and it has :the characteristic that .the woltage 'ialls pnaoticallyto zero at the resonant iireq-uency. -With :any appreciable effective resistance in the circuit, the bottom of the depression would .depart more Ir-om .linear-ityaa-nd the s oltane would not tall .near-zeroat resonance.

It mayalsoioe noted that-the :signal from the balanced abridge (circuit does not itself cause the ionization of .the-gastube, .butpreventsthe ionization'of this .tube the single-peaked control voltage illustrated Qbythe-curve H12 tin Fig. 2 excent in the immediate vicinity of resonance. As long as the voltage shown v:the curve 104 has a certain appreciable the voltage shown by the curve H12 cannotreffectionization. Although it is the voltage shown by the curve 11M that causes ionization, still-itis-the voltage from the balanced bridge circuit shown by the curve i9 2 .whichdetermineswhen .or at what frequency ionization is -.efiected. v

It should be noted ifunther that with this arl-rangement, the gas-tube cannot be ionized bya ibroad-signaL-butlis responsive ronly .to a signal of the required sharpness, .such .as that from a broadcasting station. A .broad signal results in a high value of .the woltage represented by the 4U curve 1.0.4 wherever "the voltage represented by the curve in! ischigh. ,Thus wi-thabroad signal, .the voltage lilieftectively prevents .the ionization ofthe gastubebyrthe voltage M12. The gastube will be ionized only when thesignal is so sharp that the voltage shown by .theicurve N14 is :low atthe same frequency .or atithe same time that the voltage shownibythe .curveUJZ aisahigh. The ionization of the gas tube is thus not;possibleaby broad electrical disturbances, regardless of their so intensity. On ,the .contrary, "the gas tube may readily be ionizedzbyaweak signalirom-a broadcasting station through .heavy broad electrical disturbances which cannot .ionize the gas tube. However, if the broad electrical disturbances are .heavyenough andoccur at ,exactlythe-same time .as the signal from a broadcasting station tending to ionize the gas tube,.theibroadrlisturbances may prevent .the ionization of the gas tube by the sharp signal. But .-inasmuch as .the common on varieties of hroadz electrical disturbances are exltremelylbrief in duration, theichanceof their occurrence at exactly .the right ainstant is small. .It .has been demonstrated that stop-.on-carrier sets of .the type shown in .this application will 66 ,consistently tune in weak broadcasting stations :throughextremely strong .and frequent-static disturbances.

.In another aspect, it :may be noted that stop page of thetuning condensers will-not be .efiec-ted .70 .inresponse .to .abroadeignal. it will, however, .be eftected by "a narrow: signal; in the presence of .a broad signaL-if the dormerris :of longer duration isothat there some itimeat which thecontrol vvoltage resulting :the broad signal is low and that-resulting itemthemarrow-signalis-high.

Again, stoppage will not be efiected in response to a narrow signal in the presence of an uninterrupted broad signal of sufiicient amplitude. However, since the peaks of most undesired signals, such as static, are of brief duration, the present arrangement utilizing opposing control voltages provides a very effective means for eliminating the effects of interference.

While it is preferable, in order to minimize stoppage in response to static, that the single peaked control voltage have a lower maximum value than themaxima of the double peaked voltage, and that the depression between the maxima of the latter be narrower, at least at its lower end, with respect to frequency than the upper part of the former, as indicated in Fig. 2, these are not necessary.

In the event the tuning of the receiver is carried out in one direction only, i. e., by varying the frequency in one direction, the voltage represented by only one of the two peaks of the doublepeaked curve would be efiectively utilized as the tuning means would be stopped at the res- 'onant frequency when approaching that frequency.

In Fig. 4 there is illustrated an alternating current superheterodyne receiver embodying my invention in which elements corresponding to elements of Fig. 1 are indicated by like reference characters. The receiver comprises an aerial I6,

a radio frequency tube I I, a first detector and oscillator tube I2, intermediate frequency tubes I3 and [3, a tube I4 comprising a diode section I5 and an audio amplifier section I6, an audio amplifier tube I6, and two power amplifier tubes II connected in push-pull relation. The output of the power tubes I1 is connected through a transformer I8 to the voice coil I9 of a speaker 26. En ergy is supplied by means of a power pack designated generally by the reference numeral I86, receiving current from the continuously energized power leads I01 and supplying B voltage to conductor I08 and screen grid voltage to a conductor I09. The cathode of the tube I6 is maintained at a positive voltage relative to ground by a resistance III), and a voltage negative relative to ground is supplied to a conductor I I I by means of the resistor III.

The power pack comprises a transformer II2 which supplies current of suitable voltage, for

example '24 volts, to a sensitivity selector relay I I3 and one or other of the windings 'II and I2 of a reversible alternating current capacitor mo- The receiver is tuned by means of the gang of condensers 29 operable by a shaft 30 driven by the motor 3| through pinion II4, gear I I5, clutch elements II6, pinion III, gear II8, pinion H9, and

.gear I20, the direction of drive depending upon which of the windings II or I2 is energized. At

each end of the condenser travel, an abutment member 60 carried by the shaft 30 operates limit switch BI from one position to the other so as automatically to connect the other winding II or I2, as the case may be, in circuit and thereby reverse the automatic drive of the condenser shaft. It may here be noted that when the motor is operating automatically to drive the condenser in one direction and in the other, the windings II and I2 are connected across the secondary of transformer II2 through a switch I2I, resistance I22, fixed contact blade I23, and the movable and grounded switch blade I24 by the limit switch SI, with condenser I24A being alternately connected in series with first one and then the other of the two windings. When the motor windings are energized through the aforementioned resistance I22, the motor is effective to drive only the tuning condensers, provided, of course, that the clutch elements II6 are in engagement. At this time the motor armature I3 is biased to the left, as viewed in Fig. 4, by means of a spring, and keeps out of mesh pinion I25 and gear 26 forming part of the volume control mechanism.

When it is desired to operate the volume control, to be described specifically hereinafter, the full voltage obtainable from transformer H2 is applied to either of the windings II or 12 by means of normally open button switches I21 and I28, respectively. When either of the switches I27 or I28 is closed, the energization of the motor windings is thus increased to move the armature I3 to the right, as viewed in Fig. 4, when it starts to rotate against the action of the spring to effect meshing of pinion I25 and gear I26 and rotation of the latter. The gear I26 is connected through pinion I29, gear I38, pinion I3I, and gear I32, to a shaft I33 carrying the movable contact '5'? of a volume control potentiometer 58.

The motor circuit power control switch I2I and a switch I3 i controlling the supply of power to the power pack are also operable by the motor driven shaft I33. These switches, like the switches 22 and 59 of Fig. 1, are open when the shaft I33 is at the zero volume position of the contact 51 of the potentiometer 58. When the switch I21 is closed, the winding 'II is connected directly across the secondary winding of trans former H2 and the motor 3I operates to increase the volume setting of the potentiometer 58 and to close the switches I2I and I34. The switch I34 is in series with the primary of the power transformer I35 of the power pack Hi6, so that the set is put into operation by closing the switch I21 and holding it until the switches I34 and I24 are moved to circuit closing position. When the switch I28 is closed, the volume setting of the potentiometer 58 is lowered, but the switches I2I and I34 do not open until it is lowered to zero.

One side of the secondary of the transformer H2 is grounded and connected by a conductor I36 to one side of the power and volume control switches I21, I28, a station changer switch I37, and a sensitivity selector switch I38. The other side of the switch I38 is connected by conductor I 40 to one side of the sensitivity selector relay I I3. The other side of the switch I3! is connected to the conductor I40 through a current limiting impedance MI. The other side of the switch I2! is connected through conductor I42 to winding I I of the motor 3 I, and the other side of the switch I28 is connected through a conductor I43 to the winding 12 of the motor 3I. Windings H and I2 are also connected by conductors I43 and I44 to the two fixed contacts of limit switch 6|. The other sides of the windings II and I2 are connected through conductor I46 to the ungrounded side of the secondary of the transformer H2, as is the other side of the relay II3.

The switches I27, I28, I31, and I38 are preferably assembled as a remote control unit and connected to the receiver as by a single cable I41 including conductors I36, I40, I42, and I43. This cable may be of any desired length so that the remote control unit may be located at a suitable position for convenient operation by the listener.

When the sensitivity selector switch I38 is closed, the relay I I3 is energized by the full voltage of the secondary of the transformer I I2, and when switch I31 is closed, the relay is energized the charging and discharging of the condensers which are then in circuit.

The output of tube I3 is connected to the rectifier section I19 of a tube I80 through a pair of tank circuits constructed and arranged as described in connection with Fig. 1, and the various elements of which are indicated by like reference characters. The condensers 84 and SI connected to the tank circuit are also connected to the anode of the rectifier section I19 and to the cathode common to the rectifier section and a triode section ISI. A resistance I82 is connected between the cathode and the anode of the rectifier so that, when a voltage is applied across the rectifier, a negative voltage appears at the anode end of the resistance. This negative voltage is applied through resistor I83 and a bias battery I84- to the grid of the triode section of the tube, the battery being utilized so that a higher plate voltage may be used. The cathode is connected to the control grid of the gas tube Iii through a resistance I85, the grid end 01' which is connected to ground by a condenser I35, providing a time delay in the action of the gas tube. The plate of the triode section I8 I is connected by a conductor I8! to the plate end of a resistor I88 through which plate voltage is supplied to the first intermediate frequency tube I3. A resistor I connecting the plate end of resistor I88 to ground is supplied to provide proper plate voltage to the triode section I8I. The control grid of the gas tube I0 is normally biased negatively with respect to ground by connection to the previously mentioned conductor III through a resistance I 89, which resistance is thus also efiectively in the plate cathode circuit of triode section #85 of tube I30. The gas tube and relay IEI also have in circuit therewith the condenser 98 adapted to discharge through the relay to effect fast operation of the latter, the bleeder resistance I80 to provide grid biasing current flow when the tube is deionized, and the current limiting resistor 99.

The operation of the arrangement of Fig. 4 is as follows:

When the receiver is out of operation, the motor energizing circuit is broken by the switch I2! and the power pack is deenergized by switch I 34. The sensitivity selector relay H3 is deenergized and while the switches controlled thereby may be in one of their two positions, it will be assumed they are in the positions indicated in Fig. 4, i. e., switch I48 is closed and switches I52-I53 and I54--I55 open, thereby conditioning the receiver for reception at low sensitivity. The relay ESI is deenergized so that (1) switch blades I66 and I61 are out of engagement so that the receiver is conditioned for low sensivity reception because resistance I65 is in circuit more negatively to bias the grid of tube II; (2) switch blades I68 and I69 are in engagement to ground the grid of amplifier tube I6 and thereby mute the receiver; '(3) switch blades I23 and I24 are in engagement to condition the motor 3| for operation after opening of push button switch I21, which is closed to start operation of the receiver; and i) switch blades It! and I78 are in circuit opening position to disconnect condensers I16 and ill from ground, thereby to provide the fast AVC. The clutch control relay I60 is also deenergized so that the condensers are operatively connected to the motor shaft by the clutch elements IIS.

To place the receiver in operation, volume increasing switch I2! is closed for a length of time sumcient for the motor 3| to close switches I2I and I34. Closure of switch' I21 results in the application of full voltage to motor winding II, and the motor armature I3 is displaced to the right to elTect meshing of pinion I25 and gear we and consequent rotation of shaft I33 in volume increasing direction to close switches I 2| and 34. The motor is effective also to drive the condensers as relay I60 is deenergized.

When the switch I2! is opened, the motor continues to rotate first in one direction and then the other to drive the tuning condensers under the control of limit switch BI, the motor remaining energized through a circuit including the limit switch, switch I2l, resistance I22, and

. the closed switch blades I23 and I24 of relay IGI until the tubes have been heated and a station received. When switch I2! is opened and resistance I22 placed in the motor circuit, the energization of winding H (or of winding 12, depending upon which is in circuit at any particular time) is decreased so that the volume control shaft I33 is no longer driven as pinion I 25 and gear I26 are moved out of mesh by action of the spring on the motor armature.

After the tubes have been heated, but prior to the reception of a carrier signal from a broadcast station, the plate current of tube I3 is high because no AVC voltage is applied to its grid. Consequently, the voltage drop across resistance I88, whi h is in the plate circuit of the tube, is also high. This high voltage drop results in the application of a relatively low voltage to the plate of the triode section of the tube I 80, which, it may be remembered, is supplied through conductor I81, connected to the plate side of resistance I88. Upon reception of a carrier signal the voltage applied to the plate of the triode section of tube I80 is substantially increased, and in a positive direction, because of the application of the AVG voltage (which is fast at this time, as explained above) to the grid of tube I3 and tubes preceding it.

The positive voltage thus applied to the plate of the triode section I 8| of tube I80 as a broadcast station is tuned in has a value which follows approximately the contour of curve I02 in Fig. 2. It is to be noted, therefore, that resistance E88 corresponds to and performs substantially the same function as resistance 91' of the embodiment of Fig. 1.

While the plate voltage of the triode section of tube I80 varies, just as described, with the tuning in of a station, the grid voltage of the same section is also varied in a manner now to be described. As a station is being tuned in, the tank circuits produce a voltage across condenser 82 that varies with frequency as indicated diagrammatically in the curve I03 of Fig. 2. This voltage is rectified by diode section I19 of tube I80 and is applied as a negative voltage to the grid of the triode section I 8|, by reason of the grid being connected to the plate side of the resistor I 82 of the rectifier circuit. Becauseof amplification in the triode section, this negative voltage has an effect on the plate current of tube I81 corresponding generally to that shown by curve I04 of Fig. 2.

The plate current of tube I80 thus varies with frequency, as a station is being tuned in, in a manner generally indicated by the curve I05 of Fig. 3, this curve indicating, as already described, the algebraic sum of the curves I02 and I04 of Fig. 2, and the resultant voltage applied to the grid of the gas tube 10. The plate current of tube I80 flows through resistance I89 connected enac -r 21 betweenthe cathode of the tube and ground.

Thevoltage drop across this-resistance is applied The eifectiv'e voltage thus applied to the grid as'a'result of the opposing positive iandnegative control voltages applied to the plate and grid :of the .triodesec- "-tionof tube [89, is indicated bycurve 105 of Fig. .5. Consequently, when the tuning condensers :are in a position very closelyzto .tuneain astation,

the positive voltage exce'edsthenormal negative :bias applied to the grid of the gas tube and the tube is ionized.

The ionization of the gas tube upon the tuning in of a station result inithe energization of re- ,lays 160 and I6] and theforrner at a fast rate becauseof the dischargepf condenser 98 there- .through. The relay l fifideclutchesthe condensers from the motor, thusmeventingiurther driving of them by the inertia of the motor and; associated parts, the relay I160 thusdetermining the rapidity of stoppage.

The energization. of relay Ii 6 l results in l de- .energizationof the Inotorlby disengagement of contacts I23 and l2 4;;(2) the change of the fiast AVC to a fsloW AVCby grounding condensers I15 and 111,-which-resu1tsfrom engagement of switch blades l 6landjfi8 and of blades lit-and H8, respectively; (13) renderingthe amplifier it eiTective by vtingrounding fits grit, which TeStlltS from the disengagement of blades J88 and 159 and (4) increasing the sensitivity of, the receiver by shunting out resistance. 1 6 5,1which results from engagement of switch blades tfifi, I61 and iiqfi.

Whe'nit is desired 'to "tune, in another station,

push button switch [Sigis'closled momentarily if it is desiredito' tune; in the next stationon the dial, or for a longer time if it isjvdesired. to. .tiliie', in some other station. Closurejojf switch I31 results fin theenergi'zation or relay I ['3 through jcurrent limiting impedance' l'd i sothat the relayis energized sufiiciently toop'enswitch {l4 8"but not to operate ratchet wheel 1 43. Openin'g of switch hi8 opens the --plate-cathode circuit of the'gas tube it with the result? it is deioniz'edland relays l te-and 16! are -"deenergize'd. I he rnotor 313s thereupon energized and rotates-the tunin @91 densers until-another station is tuned i'n,' in'the manner described above. v I

The volume may-be regulated at willduringre ception by closing-switch -Zfl to increase and switch Hi8 to decrease the volume These switches connect windings Hand: 1?,resbectively, directly-across the secondary 'of transformer I; so that the motor drives volume control shait B3 to increase ordecrease the volume, as heretofore described, without changihg the tuning.

In case itis desired to select high sen sitiuity, pushbutton switch 88 is closed. This connects relay winding I I3 directly across the secondary of transformer land it is energized sufiiciently not only to open 'switch 14 8 and therebycondL tion the system for-reception of another station; but also tooperate ratchet when i4 9. This wheel rotates cam i5 I from itsjindicated position to a positionto close switches 152 53 and"!,5 3,l5 5. The former shorts outresistance 65 to increase the sensitivity of tube l l, in the samemanner as is is increased upon energization of relay I6 I and the latter shorts out resistance 158 119) increase the sensitivity of gas tube-10 The "result is reception at increased sensitiv y at all timesso that both weak-and strong stations maybe received.

*As previousiy indicated, 'the high fand low' selectivity may be controlled by adjusting'the effective values of resistances ii-Tend -ifi5' contro1ling "the grid bias oftubesfj-fl and H.

When it is desired to change from high "to low 'l'selec'tivity, push-bntton' switch' -I 38 is again c osed to energize rela'yfl l'3 sufficiently to open switch 38 "arid operate ratchet wheel "i 59, after which a strong stationwill be tuned in, 'asjalreadyde- "scribed.

To render the receiver inoperative, p'ush butoperated in volume decreasing direction until power switches-l 2i and I are opened.

In theabove describedembodirnent the'proper height of the "opposing control voltages' is fobtamed-by varying the values of {the'res'istances I 4 and 175, WhiOh control the effective AVC voltage value of the double peaked'control toltage." The above adjustment of the value'of the single peaked voltage ismade possible without sacrifice of the effectiveness of theAVClby utilizingin effectfoiie AVC for tubes II, l2, and |3,aud another;ror

tube i3, thesource of thesingle peak "central voltage.

The curve lfl2 should behig h enoughito cause "the ionization of thetu-be 10 by the weake'st stations desired bearing in mind that'the'valley -between'the two peaks of-the curve [04 doesnot fall approximately to zero except on an unmodulated signal. The-height of the curve 102 should be liept sufiiciently below; the curve m4 so that the stop r m-carrier action is sumcienuy sharpv and so-that broad signals, "such as static, do not ionize the gas tiibe 10.

In Fig 5 there isillustr'ated another embodiment of my invention having a-di ffe'rent arrangement of tubes but utilizinglthe balanced'bri d g'e circuit of the embodim'ents of-'Figsi land lg which, however is illustrated physically rather than diagrammaticallyasin Fig.- 1. The elements of the balanced bridge circuit are mounted within a shielding container 106. The LmQuntin'g means consists mainly of -agrounded metallic supporting plate-200. to the under side of which is secured agenerally H-shapedbracket 2B! ofinSuIating material and on the-uppervside of whichtheie is mounted anin's'ulatin'g plate 292. Theadjustable condensers-8l and 82 are secured to the insulating-plate 202 and-the windin'gs 16 and-76' are mounted between the pIate ZGEI-and the transverse leg Of bracket-ZOI. The inductance coil 8| is mounted on a support adjustable by a screw 203 and I extending {between the; lower ends of the upright legs of the bracketllll. Condensers m and 83 and resistance- 80 arets'upported within the container-by the conductors associated therewith.

a Inthe embodiment of Fig. 5 a control voltage from the balanced bridge circuit is applied di} rectlyto the controlgrid 204 of a pentodetube 225, as is a second control voltage obtal d the AVG voltage. The voltage from th'balariced bridge circuit is applied throngh a conductoriflfi and the condenser! while-that obtained from the'AVC is appliedthroughconductorffiiirancljan denser 2H and resistance 2"] is also shunted by condenser 2I2.

The control voltage obtained from the balanced bridge circuit is, of course, alternating, but it is rectified by biasing tube 205 so that it operates near cut-off. Proper bias of the grid 284 of this tube is effected by a bias resistance 2E3 connected between the cathode of the tube and ground. The bias is normally maintained by reason of the connection of the cathode end of resistance 213 to the B supply lead 28 through a ;bleeder resistance 2I4 and current limiting resistances 215 and 216.

The plate circuit of tube 285 includes a resist- The voltage drop across this resistance is The resultant control voltage applied to the grid of gas tube '18 is indicated by curve I65 of Fig. 3, vwhich, it may be recalled, is the resultant of .curves I82 and H14 of Fig. 3. The control voltage represented by the single peaked curve N32 is the -voltage at the plate end of resistance 2H resulting from the application of a portion of the AVC ,tothe grid of tube 2H5 through conductor 26?.

While the AVG is a voltage that becomes more negative as resonance is approached, the effect on ;--the control voltage applied to the grid of gas tube '10 is to make it more positive. This results from the decrease in the plate current of tube 285 as a more negative voltage is applied to the grid. The

decrease in plate current decreases the voltage drop across-resistance 211, so that the grid of tube 70 is made more positive.

The control voltage represented by the double peaked curve I84 is voltage at the plate end of resistance 2 I1 resulting from the application of the output of the balanced bridge circuit to the grid of tube 285. When a station is being tuned in, the

output rises gradually to a maximum and then falls sharply to a minimum at resonance frequency, which may be zero if the cricuit is properly balanced. Since tube 205 is biased to near cut-01f, it acts as a rectifier and the plate current of the tube rises gradually to a maximum and then falls sharply to a minimum. This variation in plate current, which flows through resistance 2H, causes the plate end of the latter and the grid of gas tube 10 to be more negative and then sharply less negative as a station is tuned in, the v,

grid potential being represented, as already indicated, by curve I84.

In this embodiment, the height of the single peaked relative to the double peaked curve is adjusted by properly proportioning resistances 289 and 2 I0. This relation is maintained irrespective of whether the station be strong or weak, as both the single and double peaked control voltages are amplified about the same degree in tube 295. The separation of weak from strong stations by the sensitivity control is, therefore, made more effectively.

The operation of the embodiment of Fig. 5 when a station is being tuned in is substantially the same as that of Fig. 1, so that it is not deemed necessary to repeat it at this point.

Another balanced bridge circuit embodying the principles of the present invention is illustrated in Fig. 6. This circuit may be used with the par ticular circuit arrangements of any of Figs. 1, 4,

and 5 by substituting the elements enclosed iri dotted lines for similarly enclosed elements in the latter figures. The primary l6 and secondary 18 of transformer 11 are connected to the circuit as previously described. A first tank circuit, tuned to intermediate frequency, is constituted by the primary winding, and adjustable condenser 226, an inductance comprising two parts 222 and 223, condenser 224, and the condenser 18. Portion 222 is made of comparatively few turns that are adjustable to vary the mutual inductance therebetween and an inductance 22 6 forming part of the second tank circuit. This construction provides, in effect, a Vernier adjustment for the mutual inductance. A second tank circuit, also tuned to intermediate frequency, is constituted by the condenser 224', the inductance 228 coupled to inductance 222223, and an adjustable condenser 228. The junction of condensers 224 and 228 is grounded and the junction of inductance 225 and condenser 228 is connected to the tube to be supplied with the output from the balanced bridge circuit through the condenser 84.

The two tank circuits are coupled inductively by reason of the coupling between the inductances 222-223 and 226 and capacitatively through condenser 224. The mutual inductive reactance of inductances 222223 and 228 and the capacitative reactance of condenser 224 are so adjusted that they are equal at intermediate frequency and the voltages across inductance 226 and condenser 224 are brought into exact phase opposition by a resistance 230 and a very small adjustable condenser 232 connected across condenser 228 and inductance 222223. Considered in another light, the electromagnetic and electrostatic couplings are balanced or neutralized, i. e.,

the inductive and capacitative reactive components are made to balance or neutralize. There remains, however, a resistive component of coupling which can be balanced or neurtalized only by a resistive component supplied, in the arrangement of Fig. 6, by the resistor 230.

The arrangement, like that of the previously described circuit, is such that at intermediate frequency the voltage across condenser 224 is equal to and opposite the voltage across inductance 226. In other words, there is zero effective coupling between the two circuits and zero voltage across the condenser 228.

It may be Well at this point to describe briefly how the above described balanced bridge cir-uits are adjusted. In the circuit of Figs. 1, 4, and 5, the condensers 81 and B2 of the two tank circuits are first adjusted to tune these circuits to intermediate frequency. The mutual inductance between the primary coil 16 and the secondary coil BI is then adjusted by means of the screw 283 shown in Fig. 5 to bring the steep depression or valley between the two peaks close to a minimum voltage, which should be close to zero on an unmodulated signal at intermediate frequency. Condensers 8| and 82 may then be readjusted to insure that both tank circuits are tuned to the intermediate frequency. The mutual inductance may then again be adjusted, as described above, to bring the two peaks of the curve I04 in Fig. 2 to approximately the same height on each side of the intermediate frequency. The resistance should be maintained with close enough tolerance to insure the reduction of voltage across the condenser 82 to near zero at resonance. If the values of the circuits are correct, the adjusting screw should be adjusted for zero nae new a'nsted by firstadjusting condensers 220' and-228 to tune the two-tank circuits to intermediate fredi ency. The--1mitnal coupling between the yernier inductance-22% and the inductance 226 is-then ada'usted to bring" the two peaks on-either side of the intermediate frequency to approximately the saine height. The condenser 232 is adjusted ion-correct phasing, which, when gttained will bring' the-bottom of the steep "deinression-between the two peaks close to zero-voltage. The above-- described adjustmentsare all rlativelyindeiaendent ofone another, the capac ity, o;,f,- c9nde n ser, zggbeingscasmall as to have a ngll ible eflfcct lnitlie, l ning of. the tank circuit.

'Tneiaggro rirnate-val sot; the, circuit constants tenths. ew oil 460': ke l-are as. follows:

.A fnrtheremhodiment: of hebalanced ridge circuit.isdllustratedin:fies 7 and 8. It 1s m many-respectssimilantcthe. embodiment f Fi 6 andamayebe used: in: place of: the; la ter by substitutin the circuit; enclcsed-vbva-d tted lines-J 1n Eli's; l ior thetzenc csed within: the dottedi i of Fig. 6. 1

The. nrimarr and secondary windings l6 and 1 !.:are qonnectedtothe cirouites-p l e- .scribed: T ebrideecircuitinciudes coils :40 and 2L4? adiiista ly coupled. t eacnother in a manner he e na terito be described in detail. These cells haveamutnal.lnductancersimilar to that bee tweencoils 2 22,112} andllfi Qflfigfi, Connected inserie withco i. m ca we t e inductance df ewhichisalso. adiustable andthe. value. of which isL-large relative to; the total inductance of coil 242.. whereby ash stmenteither mutu l induct-- ance; between I coils, 22 41}. and 24,2 can be efiected withibnteslightdetnning;or;coil.244; This results from he f ctzthai esen arelat vel-ylarge chan v or i he mu u l. n uctance beiw ens cons 240. and

l-fl results in only a small percentage change in rcuit ot-Fig, lifter-an intermediate frethe total secondary imam-stance whichfequals the s mor'th inductances of coils 2'42and' 244. The balancedbridge transformer 'shownfin Fig. '7 utilizes a movable powdered iron core I215 providing apermeability tunedinutual inductance instead 'ofthe vario'm'e'ter itypeloftuning used'inthe transformer shown inFigffi'; The movablecore "is also a convenient adjustment for the primary and secondary and eliminates variable condensers.

Another diff erenc'ebetwenthe balancedibrid'ge transformer shown in Fig. land that disclosed in'Fig. 6 'is in the 'nlialsingjor resistance neutralizing arrangement which comprises the adjustable con-denser nd resistors 25c and 252. These three components are similar to the series resistor "230. and the condenser 232"ofFig. 6, but divide the totalresis'tance in the phasing network between two parallel; branches, one of which is 'ri-otcanacitative. "This permits a finer, v'ernie'rlik'e phasing adjustment if theproper values of resistance and capacity such as those indicated hereafter, are chosen. It willbe understood that the capacity 248 fnerely for providing a means for varying the effective resistanceas a suitable variable" resistor isjdimcult to make.

The circuit also includes condenser 254', 256, and!fililcorrespondingto condensers 220 ,1224, and 22K of'Fi'g. '6 exceptthat condensers 2'54 and 256 are not adjustable. The additional'condensers "l8 and fidcorresponjd to. the like numbered condensers'o'f Fig. 6,; 7

Fig.8 illustrates; a practical-balanced bridge transformer embodying the. circuit elements dis closctl'in Fig. 7. Thelinductances capacities needing adjustment, have beenillustrated in their proper. relationship, The various 'fi'xed condensers, resistors andithe, associated wiring and leads are omitted in order not to obscure the structure. The primary andsecondary windings l6! and I76, the coils 24D andf242, and the indnctance coil Zllare provided with movable internal cores 2B0, 246lai1d. 262, respectively, to effect permeabilityitiinlng. Externally projecting slotted-he'ad screWsEZjG IQZGB, and (268 are provided ion the cores 260, 136 and '262, respectively, so thatscreW-driver adjustment. of the inductances can be accomplished-without removing. a transformer shield 210. It. should be noted that the coils Hill-and 242;areoriented atright angles to the, primary coil "I5 andthat coil 244 is similarly disposed relative tocoils 240 and 242 in order to minimize the inductive coupling between the coil fields. V

The variable phasing condenser- 248 is located between the tubular-supports 2-12 and 214 for coils 240, 242 and-2,4,4 which also serve as guides for cores-246 and 262,.respectively. It consists essentially of an insulating bracket 216, a flanged, cylindrical condenser stator 218 mounted thereupon, a movable cylindrical condenser electrode 280 of slightly smaller diameter than the inside diameter of the stator movable.axially therewithin and maintained in concentric relationship therewith by meansof aminuteannular insulater 282 encircling-the end ofrthe electrode". The electrode is arranged for; axial movement by means of an externally adjustable slotted-head screw 284, similar to those usedufor adjusting the inductance cores;

It-maybe well to, review the adjustment of the balanced bridge transformers with particular reference to the embodiment of Fig. '7. The process o adjustmentiisr, ingeneral, similar for all varieties: of transformer; of the type disclosed in this application. There are, however, specific 23 differences resulting from the use of different circuit components for effecting the adjustment and from the greater or lesser degree of independence between adjustments realized in the several embodiments described.

The adjustment of the transformer is effected using a signal generator delivering an alternating. current voltage of a predetermined frequency, which may be the intermediate frequency, to the grid of the intermediate frequency amplifier tube preceding the primary 16.

When the transformer is yet far off balance, adjustment must be carried out while measuring the voltage across the primary winding 76, preferably by means of an A. C. vacuum tube volmeter. The core 266 is adjusted by means of the screw 264 to give' maximum primary Voltage, which corresponds to primary resonance. The inductance coil 244 is then tuned by adjusting its core 262 with screw 268 until the primary voltage is at a minimum. This point corresponds to secondary resonance with a high degree of coupling to the primary. The mutual inductance is then varied by adjusting movable core 246 with the screw 266 until the primary voltage again reaches a maximum. This maximum is a result of decreasing the coupling to the secondary which reduces the secondary load upon the primary and permits its voltage to rise. The point of maximum primary voltage and minimum coupling corresponds to the dip frequency and the dip is thus made to coincide with the predetermined frequency by this adjustment. The adjustment of. the mutual inductance results in a noticeable, though comparatively slight, detuning of coils 16 and 244, which may be retuned.

Upon the completion of a preliminary adjustment, as described, the coupling between the primary and secondary coils 240 and 242 may be reduced very nearly to zero by adjusting the phasing condenser 248 with the screw 284. Further slight adjustment of the mutual inductances of coils 240 and 242 may also be made. Now, it will be found that tuning the secondary coil 242 no longer affects the primary voltage. At this point it is appropriate to measure the signal voltage at the output of the transformer across the secondary condenser 258. The exact position of the dip can then be adjusted by varying the mutual inductance core 246 with the screw 266 and the secondary may then be tuned to give peaks of equally high voltage to either side of the dip with-out influencing the position of the dip. The final step in the adjustment of the transformer is to manipulate the phasing condenser 248 by means of the screw 284 to yield a minimum output voltage at the dip, which may be made to approach a value which is practically zero.

The effect of the various adjustments and the circuit parameters and couplings which they control may be summarized as follows:

Adjustment screw 264 tunes the primary circuit to the intermediate frequency. Adjustment screw 266 adjusts the frequency of the dip to the desired frequency, which may be the intermediate frequency or some frequency close to the center of the intermediate frequency band. Adjustment screw 284 balances or neutralizes the resistive components unavoidably associated with the circuit reactances by introducing additional resistive components by means of resistors 250 and 252, which components buck or oppose, and thus neutralize the resistive coupling between the primary and secondary at the dip frequency.

24 Thus, zero coupling between the primary and secondary circuits and zero ,voltageacross the secondary condenser 258 at,the dip frequency is attained. The two peaks on each side of the dip are brought to their. maximum height and made generally equal in height by adjusting the secondary tuning screw 268. In this way there is pro.- vided a response curve with two high peakson each side of a deep, narrow valley going down practically to zero voltage.

Fig. 9 illustrates the output voltage curve of a transformer ofthe type disclosed in Figs. 7 and 8 obtained fromactual laboratory measurements of a transformer incorporating the values of circuit components listed hereinafter.- The limitations of reproduction do not permit exact illustration to scale of how close the dip approaches to zero, because when closely adjusted, the difference between the voltage at the clip and zero is not readily discernible upon this scale of reproduction.

The following are the values of the circuit parameters used in this embodiment of the invention to obtain a dip frequency of 455 KC.

Inductance l6 mh 439 Inductance l6 mh .71 Inductance 246 mh .266 Inductance 242 mh .266 Inductance 244 mh .69 Mutual 242-244; mh .09 Condenser 24B ML. .5 Resistance 256 ohms-.. 150,000 Resistance 252 megohm-.. 1 Condenser 254 I. mmf.. 113 Condenser 258' mmf 83 Condenser 256 ML. 700

The balanced bridge circuits described above, while of proven merit, are intended merely to be illustrative of the principles of the invention and they may be modified by those skilled in the art to meet the exigencies of the particular installation or use. The primary principle embodied in the circuits is the obtaining of a reduced output voltage at a desired frequency which can be reduced substantially to zero by coupled circuits charac terized by capacitative and inductive reactances brought into proper phase opposition, and furthermore where the output voltage is sharply decreased at the desired frequency. While proper phase opposition has been described as being obtained by adding resistance to the circuit or a resistance and a capacity controlling the effective value of the resistance, it is possible to obtain the phase opposition by utilizing a reactance element having a resistance component. Other modifications of the invention may be made without departing from the principles set forth herein.

What I claim as new, and desire to secure by Letters Patent of the United States, is:

1. A frequency discriminating apparatus'having an input circuit and an output circuit, and a coupling network between said input and output circuits including two inductances coupled together and connected to each other at one end, a first capacity, one side of which connects to the junction of said inductances and the other side being common to said input and output circuits, a second capacity in series between a first of said inductances and the input circuit, resistive means shunted across said second capacity and the first inductance, a third inductance in series with the second of said inductances, and a third capacity shunted across said second and third inductances and first capacity, a movable ferro- 25 magnetic core movable relative to said first and second inductances to vary the coupling therebetween, and a movable ferromagnetic core movable relative to said third inductance to tune the output circuit.

2. A frequency discriminating coupling apparatus comprising a primary and two secondary circuits, a single shielding container housing said circuits, one of said secondary circuits including a coil mounted with its axis parallel to the axis of said primary and the other secondary circuit including a coil mounted with its axis perpendic ular to said primary, inductive and capacitative means coupling said primary and said perpendicularly positioned secondary circuits, said inductive coupling means including a stationary coil of wire and a ferromagnetic core movable relative to said coil to vary the inductive coupling component so that the algebraic sum of the inductive and capacitative couplings between said two circuits is substantially zero at a predetermined frequency, and separately controllable resistive means coupled to said inductive coupling means for substantially reducing the algebraic sum of the resistive couplings between said circuits at said predetermined frequency, separately adjustable reactive means for tuning said primary circuit to said predetermined frequency, and separately adjustable reactive means for tuning said 26 perpendicularly mounted secondary circuit to said predetermined frequency, said core, separately controllable resistive means and the two separately adjustable reactive means being located in said container and adapted for convenient adjustment from outside said container.

WILLIAM J. OBRIEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,577,421 Hazeltine Mar. 16, 1926 1,646,364 Spencer Oct. 18, 1927 1,925,160 Whitelock Sept. 5, 1933 1,953,465 Chestnut Apr. 3, 1934 1,970,933 Frederick Aug. 21, 1934 1,982,690 Polydorofi Dec. 4, 1936 2,044,645 Stapleton June 16, 1936 2,063,295 Braden Dec. 8, 1936 2,093,655 Tellegen Sept. 21, 1937 2,096,874 Beers Oct. 26, 1937 2,098,331 Bowman Nov. 9, 1937 2,231,806 Goldsborough Feb. 11, 1941 2,262,218 Andrews Nov. 11, 1941 2,353,180 Muller July 11, 1944 

